If your source equipment has balanced (differential) outputs then use the balanced input connection to the SA400.

The real advantage of balanced inputs comes from the fact that two signals are provided that are both 180 degrees out of phase with each other, hence, any extraneous signal e.g. RF, hum, or noise will be cancelled out by the common mode rejection of the input stage. This is particularly Important with long (5 metres or more) input cable runs. If your source equipment is (as is normal) located close to the amplifier then the phono input will be totally adequate and will achieve the best performance.